1. Field of the Invention
The present invention relates to an antenna tuning circuit for an AM radio receiver, and more particularly, to an antenna tuning circuit for long and medium wave AM radio receivers.
2. Description of the Prior Art
A conventional antenna tuning circuit for an AM radio receiver is provided with a parallel connection of an antenna coil wound around a ferrite core and a variable capacitor. A tuned output signal from the tuning circuit is applied to a first stage transistor through a secondary coil. In such a construction, it has been found that the secondary coil acts as an antenna for signals short wave, and, therefore, the antenna tuning circuit undergoes interference from the short wave signal.
The ferrite antenna is usually mounted on the outside of a chassis, so that a signal from a hot or energized side terminal of the antenna tuning circuit must be fed into the circuit board in the chassis by a lead wire. The path of the lead wire for leading the signal into the circuit board can catch external noise since the impedance of the tuning circuit is extremely high at its tuning frequency. This fact leads to an extreme deterioration of the signal to noise ratio of the receiver.
Accordingly, an object of the invention is to provide an improved antenna tuning circuit which may overcome the foregoing disadvantages of the conventional antenna tuning circuit.
In order to achieve the above object, the present invention employs a field effect transistor which is free from current noise in principle and which has a high input impedance in such a way that the hot side of an antenna coil is directly connected with the gate electrode of a field effect transistor (hereinafter merely referred to as FET) connected in a source-follower circuit configuration. With this circuit arrangement, the secondary coil becomes unnecessary for the antenna tuning circuit, so that the problem of the current noise of the bipolar transistor as well as the adverse effect of the short wave signal received by the secondary coil may be solved.